1. Field of the Invention
The present invention relates to a carotid-artery-plaque ultrasound-imaging method, a carotid-artery-plaque evaluating device, a carotid-artery-plaque evaluating program, and a computer-readable storage medium that can noninvasively evaluate the properties of plaque in carotid artery by using ultrasound examination (ultrasonic pulse).
2. Description of the Related Art
Diagnosis of carotid artery plaque tissue characteristics can be used not only to evaluate stroke risk but also to predict ischemic heart disease events or to estimate therapeutic effect of lipid lowering drug for dyslipidemia. In particular, for diabetes mellitus, this diagnosis is important to estimate the prognosis or QOL of a patient.
Presently, intravascular ultrasound examination (IVUS) is used as invasive diagnosis of carotid artery plaque properties. However, the intravascular ultrasound requires catheterization in which a catheter type ultrasonic probe is inserted into a blood vessel. For this reason, the intravascular ultrasound cannot be performed for everyone. Accordingly, patients needs hospital care. As a result, it is difficult to repeatedly conduct the intravascular ultrasound.
Contrary to this, CT and MRI are used for noninvasive diagnosis of the properties. However, there is a problem with radiation exposure in the case of CT, while there is a problem that the resolution is poor in the case of MRI. In addition, CT and MRI require a large-scale device. For this reason, these noninvasive diagnosis methods cannot be conducted in a small clinic. Also, outpatients cannot freely have these noninvasive diagnosis methods.
On the other hand, a carotid artery ultrasound examination that uses echo is also used to evaluate the properties of plaque that is included in carotid artery. It is said that tissue property diagnosis can be conducted based on the echo plaque properties by the carotid artery echo test. In general, it is said that echolucent plaque near the intensity of blood shows atheroma or hematoma, that isoechoic plaque near intima media or muscle shows fibrosis, and that hyperechoic plaque near bone shows calcified lesion. Cerebrovascular infarct risk can be predicted by diagnosis of plaque based on the level of intensity. Also, risk of coronary event coronary artery can be predicted by diagnosis of plaque based on the level of echo intensity. The carotid artery echo test is a noninvasive test which outpatients can easily have.
It is considered that the echo intensity of the carotid artery plaque provides observations of diseased tissue in arteriosclerosis (Non-patent Literature 3). Generally, plaque shown by low echo intensity in echo image richly contains lipid or macrophages, and is referred to as vulnerable (unstable) plaque. On the other hand, plaque shown by high echo intensity is plaque in an advanced stage of fibrosis or calcification, and is referred to as stable plaque. FIG. 1 is a graph showing comparison of occurrence frequencies of coronary artery disorder (event) in a few tens of years between the case where plaque shown by low echo intensity is included in echo image or not. As shown in this graph, it is reported that the event more likely to occur in the case where the low echo intensity plaque is included in the carotid artery relative to the case where the high echo intensity plaque is included. That is, it is said that, if a large amount of low echo intensity plaque is included in carotid artery, this means vulnerability or instability of coronary artery (Non-patent Literature 4).
However, the carotid artery echo test is not a quantitative test but a qualitative rest. For this reason, the test observation result will vary in accordance with the skill of evaluator. Accordingly, even the same plaque may result in different diagnosis results depending on evaluator or testing devices. As a result, there is a problem that it is difficult to properly evaluate tissue characteristics of the plaque (Non-patent Literature 1). In particular, since the echo image obtained by echo devices (ultrasonic diagnostic devices) is a gray scale image, plaque properties is evaluated based on discrimination in the gray scale echo image with eyeball in the carotid artery echo test as discussed above. However, when plaque properties are classified with the naked eye based on the echo intensity levels displayed in gray scale, since the criterion of intensity level discrimination is blurry, it is difficult to quantitatively evaluate plaque properties. For example, in the case where evaluators adjust the gain, gray scale intensity level, or contrast of echo image depending on their own judgment or preferences, even when the echo images of the same patient are obtained by the same echo device, the intensity will vary in accordance with the evaluators. For this reason, it is difficult to compare images with each other. As a result, there is problems that time-variation of a patient cannot be properly checked, and that patients' cases cannot be properly compared with each other. In addition, it is difficult to evaluate plaque properties in severely calcified lesion, high grade lesion such as high grade internal carotid artery lesion, tortuous vessel, or the like.
The ultrasound intensity of the plaque can be changed in accordance with gain adjustment or image adjustment function of an echo device. For example, as shown in FIG. 17, in the case where the gain of an echo device is set to 45, 50, and 66, the echo intensity of plaque is correspondingly changed as shown in FIGS. 17A, 17B, and 17C. For this reason, even when echo images of the same subject are obtained, the echo intensity levels will be different from each other. It can be understood that difficult to quantitatively evaluate plaque properties based on echo intensity.
The gray scale in conventional echo images is represented by the intensity value of each pixel of an echo image. However, when an echo image is produced, the echo image is subjected to a number of filtering processes. The intensity value varies depending on echo devices for measuring an echo. For example, as shown in FIG. 18, an RE signal waveform of ultrasonic reflection signal (integrated backscatter: IBS), which is raw data (RAW data) obtained by an ultrasonic probe, is amplified, rectified, demodulated, subjected to pulse compression, further amplified, and converted into its intensity value. These filtering processes are differently performed depending on echo devices. Accordingly, there is a problem that even the same RF signal will be represented by different levels in gray scale in the produced echo image.
To address this issue, a method as an example for improvement of carotid artery echo test has been developed which uses hydrazide conjugate as an imaging agent, and detects and monitors the properties of plaque (Patent Literature 1). Also, improvement of echo device, improvement of imaging process, and the like can be considered. However, these improvements are not succeeded up to now.
On the other hand, IVUS that is used not in carotid artery tests but in heart coronary artery tests is an invasive method in which a catheter is inserted into a blood vessel. However, since IVUS analyses an echo wave that is directly reflected in the blood vessel, a good echo reflection wave can be obtained so that a good echo image can be produced. Form this viewpoint, various imaging methods have been studied (Patent Literature 2, Non-patent Literature 2).
However, if these methods are adapted to noninvasive carotid artery analysis, an echo reflection wave passes through skin or tissue. Since the echo reflection wave is analyzed, it is difficult to simply adopt these methods to noninvasive carotid artery analysis.